There is a high level of industrial demand for plastics films, e.g. biaxially oriented films comprised of polypropylene or of polyester and having defined surfaces and optical properties. Films with uniform, moderate surface roughness are of interest in specific applications.
U.S. Pat. No. 3,154,461 claims a process for production of a biaxially oriented film comprised of thermoplastic (e.g. polyethylene terephthalate, polypropylene) with a matt surface, in which the film comprises non-compressible particles (e.g. calcium carbonate, silicon dioxide) whose size is from 0.3 to 20 μm and whose concentration is from 1 to 25% by weight. That application also claims the matt film produced by the inventive process. The film produced by the inventive process is too rough for many applications.
EP-A-1 197 326 describes a matt, biaxially oriented polyester film which comprises from 1 to 10% by weight of particles whose average particle size is from 2 to 5 μm. The average roughness Ra of these films is in the range from 150 to 1000 μm. No statements are made concerning the uniformity of roughness or the height of the highest elevations.
EP-B-0 152 265 describes a polyester film which comprises from 0.01 to 1.0% by weight of silicon dioxide and/or titanium dioxide whose average particle size is from 0.01 to 0.5 μm and from 0.02 to 0.5% by weight of calcium carbonate whose average particle size is from 0.04 to 0.48 μm. The small particles give this film very low roughness, but no statements are made concerning transparency.
JP-A-62 241928 describes a film which comprises titanium dioxide, silicon dioxide, and an optical brightener. The concentration of the titanium dioxide is in the range from 8 to 20% by weight, giving the film very low transparency.
EP-A-0 723 565 claims a polyester film which comprises from 1 to 20% by weight of titanium dioxide and from 0.1 to 6% by weight of mica whose average particle size is from 3 to 40 μm. The large amount of titanium dioxide gives excessively low transparency.
U.S. Pat. No. 5,403,879 claims a polyester film which comprises from 5 to 25% by weight of titanium dioxide, from 0.1 to 3% by weight of silica whose average particle size is from 1 to 5 μm, and from 0.02 to 1.0% by weight of a fluorescent organic pigment. The high concentration of titanium dioxide gives the film very low transparency. The examples mention films whose transparency is smaller than 2%, at a film thickness of 188 μm.